Fig. 2

Segregation of alleles in a three-factor cross in yeast (Saccharomyces cerevisiae). The diploid yeast cell (eight DNA strands after DNA replication, left side) produces four haploid spores at the end of meiosis, each of which contains two of the eight DNA strands. The four individual spores from one cell germinate and give colonies after repeated mitotic cell divisions (white or red circles). The three genes A, B, and C give rise to readily assayed colony phenotypes. In particular, B gives a white colony while b gives a red colony. Three possible outcomes from spore germination (tetrad analysis) are shown on the right side. Top – a recombination event (crossover) occurred between A and B in one homologous chromosome pair during meiosis. There are two red and two white colonies because two chromosomes (four DNA strands) carry B and two chromosomes (four strands) carry b, known as 4:4 segregation. Middle – A B allele has been converted to a b allele so that there are three red colonies and only one white colony from the tetrad. There are thus six DNA strands carrying b and only two that carry B, referred to as 6:2 segregation. Bottom – a sectored colony has appeared indicating the presence of heteroduplex DNA in one spore. One DNA strand of a dsDNA chromosome carried B while the other strand carried b. The two DNA duplexes from the first mitotic cell division after this spore germinated had either B or b in both strands, and all progeny from those original daughter cells were either B in both strands (white cells) or b in both strands (red cells), giving rise to a sectored colony that is half red and half white. The tetrad analysis indicates five b and three B alleles or 5:3 segregation